1. Field of the Invention
The present invention relates to an anti-skid brake control system for a vehicle.
2. Description of Related Art
It is often experienced that, when an automotive vehicle is abruptly braked or when an automotive vehicle is braked while running on a slippery surface such as a snow-covered ground surface, the automotive vehicle tends to skid as a result of the wheels being locked. Once this occurs, the automotive vehicle requires an increased braking distance over which the automotive vehicle runs or skids before it stops completely.
An anti-skid brake control system, often referred to as ABS, is known as a system for improving the braking performance of an automotive vehicle, including a reduction in braking distance, in such a way as to control the application of a braking force to automotive wheel brakes to avoid the occurrence of such a skid. The anti-skid brake system now in use is available in various types, one of which is disclosed in for example, Japanese Unexamined Patent Publication (kokai) No. 102,363/1986.
According to the above mentioned patent publication, the anti-skid brake control system comprises a brake control means operable to control a liquid pressure to be applied to a wheel brake cylinder used for each vehicle wheel for applying a braking force thereto, a wheel speed detecting means for detecting the rotational speed of each of the vehicle wheels, and a controller operable to calculate the slip ratio of the rotational speed of each vehicle wheel relative to the vehicle running speed in reference to respective signals supplied from the wheel speed detecting means and then to output a control signal, necessary to avoid a possibility of the associated wheel being locked, to the brake control means in dependence on the calculated slip ratio.
It has, however, been found that the use of the prior anti-skid brake control system of the above described construction often brings about the following problem. The anti-skid brake control system disclosed in the above mentioned patent publication operates in such a manner that, when an incipient skid condition occurs in which the vehicle wheels being braked exhibit a tendency of being locked, the anti-skid brake control system is brought into operation to reduce the braking force being applied to each wheel brake cylinder. The wheel speed is lowered to a certain extent by the effect of an inertia even after the anti-skid brake control system has been brought into operation, and thereafter the wheel speed increases to a value approaching the vehicle running speed. Should the inertia moment of a power train through which a drive of an automotive engine is transmitted to the vehicle wheels be relatively large (such as occurring, for example, when a clutch interposed between the automotive engine and an automotive transmission is coupled to transmit the drive of the automotive engine to the automotive drive wheels, or when in the case of a four-wheel drive (4-WD), or tandem drive, vehicle a 2-WD/4-WD changeover clutch disposed in the power train is set in a 4-WD mode), a relatively large resistance from the power train acts, when the wheel speed restores to a value substantially equal to the vehicle running speed, in a direction tending to suppress the restoration of the wheel speed and, therefore, the complete restoration of the wheel speed requires a relatively long time.
Japanese Unexamined Patent Publication (kokai) No. 43,355/1987 discloses the requirement for the four-wheel drive, or tandem drive, vehicle equipped with the anti-skid brake control system to satisfy. According to this patent publication, it is described that, when the braking is desired, the 2-WD/4-WD changeover clutch in the power train has to be switched over from the 4-WD mode to the 2-WD mode before the anti-skid brake control system is brought into operation. However, the system disclosed in this patent Publication has a problem in that, since the braking requires to be immediately followed by the changeover from the 4-WD mode onto the 2-WD mode with the inertia moment of the power train lowered consequently, the wheel speed tends to be abruptly reduced consequent upon the braking, causing the vehicle wheels to be susceptible to a locked condition.